Lithographic apparatus, immersion projection apparatus and device manufacturing method

ABSTRACT

A lithographic apparatus comprises a substrate table to hold a substrate and a projection system to project a patterned radiation beam onto a target portion of the substrate. The lithographic apparatus further comprises a fluid supply system to supply a fluid between the substrate and a final lens element of the projection system as well as a position controller to control a position of the fluid supply system. The position controller is configured to, being provided with a position quantity of the substrate, determine a desired position of the liquid supply system by adding a position offset to the position quantity of the substrate, and position the liquid supply system according to the desired position. The position quantity may comprise a position and/or rotational position of the substrate table and/or a height function of a substrate height.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus, an immersionprojection apparatus and a method for manufacturing a device.

2. Background of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,comprising part of, one, or several dies) on a substrate (e.g., asilicon wafer). Transfer of the pattern is typically via imaging onto alayer of radiation-sensitive material (resist) provided on thesubstrate. In general, a single substrate will contain a network ofadjacent target portions that are successively patterned. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an entire pattern onto the targetportion at one time, and so-called scanners, in which each targetportion is irradiated by scanning the pattern through a radiation beamin a given direction (the “scanning”-direction) while synchronouslyscanning the substrate parallel or anti-parallel to this direction. Itis also possible to transfer the pattern from the patterning device tothe substrate by imprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g., water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g., quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in WO 99/49504, herebyincorporated in its entirety by reference. As illustrated in FIGS. 2 and3, liquid is supplied by at least one inlet IN onto the substrate, forexample along the direction of movement of the substrate relative to thefinal element, and is removed by at least one outlet OUT after havingpassed under the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

Another solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. Such a solution is illustrated in FIG. 4. Theseal member is substantially stationary relative to the projectionsystem in the XY plane though there may be some relative movement in theZ direction (in the direction of the optical axis). A seal is formedbetween the seal member and the surface of the substrate. In certainembodiments, the seal is a contactless seal such as a gas seal. Such assystem with a gas seal is disclosed in European Patent Application No.03252955.4, hereby incorporated in its entirety by reference.

In yet another solution as depicted in FIG. 5, a reservoir 10 forms acontactless seal to the substrate around the image field of theprojection system so that liquid is confined to fill a space between thesubstrate surface and the final element of the projection system. Thereservoir is formed by a seal member 12 positioned below and surroundingthe final element of the projection system PL. Liquid is brought intothe space below the projection system and within the seal member 12. Theseal member 12 extends a little above the final element of theprojection system and the liquid level rises above the final element sothat a buffer of liquid is provided. The seal member 12 has an innerperiphery that at the upper end, for example, closely conforms to theshape of the projection system or the final element thereof and may,e.g., be round. At the bottom, the inner periphery closely conforms tothe shape of the image field, e.g., rectangular though this need not bethe case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the seal member 12 and the surface of the substrate W. The gasseal is formed by gas, e.g., air or synthetic air and in certain cases,N₂ or another inert gas, provided under pressure via inlet 15 to the gapbetween seal member 12 and substrate and extracted via first outlet 14.The overpressure on the gas inlet 15, vacuum level on the first outlet14 and geometry of the gap are arranged so that there is a high-velocityair flow inwards that confines the liquid.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two stages for supporting the substrate.Leveling measurements are carried out with a stage at a first position,without immersion liquid, and exposure is carried out with a stage at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one stage.

According to the state of the art, the liquid supply system may beguided with respect to the substrate by way of an air bearing. The airbearing provides for a guiding of the liquid supply system with respectto the substrate, and for a distance between a surface of the substrateand the liquid supply system, or at least the reservoir thereof.

An alternative known in the art is to position the liquid supply systemby way of an actuator instead of the guiding by an air bearing. Theactuator is commonly driven by a control device providing for apositioning of the liquid supply system. In operation, the substrate ispositioned such that its surface is kept in a focus plane of theprojection system of the lithographic apparatus. Hence, according to thestate of the art the liquid supply is positioned at a certain heightwith respect to the focus plane, to leave a predetermined gap betweenthe liquid supply system (or at least the reservoir thereof) and thefocus plane. As the substrate is positioned such that its surfacecoincides as good as possible with the focus plane, this will result ina distance between the surface of the substrate and the liquid supplysystem or at least the reservoir thereof, which is substantially equalto the gap between the focus plane and the liquid supply system (orreservoir thereof).

A problem associated with the positioning of the liquid supply system asdescribed above occurs because of an unflatness of the substrate. Tocope with unflatness of the substrate, the substrate table andpositioning system thereof is according to the state of the artconstructed to tilt the substrate table as to keep a part of thesubstrate which is to be illuminated at a certain moment locally infocus, thus locally coinciding as good as possible with the focus planeof the projection system. Dimensions of the liquid supply system arehowever significantly larger than a target portion of the substratewhich is to be irradiated by way of the projection system. As a resultthereof, a distance between an edge of the liquid supply system and thesurface of the substrate may vary significantly, in practicalimplementations a variation in an order of magnitude of 30 micrometershaving been observed. Such a variation in distance between the liquidsupply system and the surface of the substrate may result in a crash ofthe liquid supply system against the substrate or may result in a toolarge distance causing leakage of the immersion liquid.

In addition to the above adverse effects, a further problem will occurat an edge of the substrate. In common state of the art implementations,the substrate, when being positioned on the substrate table, issurrounded by a structure which may comprise sensors, closing discs,etc. As explained above, a dimension of the liquid supply system may besignificantly larger than a dimension of the target portion of thesubstrate, which is to be irradiated at a certain moment in time. Thus,when a portion of the substrate is to be irradiated, which is near anedge thereof, the liquid supply system will partly overlap with thestructure(s) surrounding the substrate when positioned on the substratetable. A tolerance in the thickness of the substrate may thus result ina height difference between the surrounding structure and the surface ofthe substrate, hence resulting in a distance between the surroundingstructure and the liquid supply system which may be larger or smallerthan a distance between the target portion of the substrate which is tobe irradiated and the liquid supply system. Similarly to the distancedeviation as described above this may result in crashes in case of adistance which is too small or leakage of the immersion liquid in casethe distance is too large.

SUMMARY OF THE INVENTION

It is desirable to provide an improved positioning for the liquid supplysystem.

According to an aspect of the invention, there is provided alithographic apparatus including a substrate table constructed to hold asubstrate, a projection system configured to project a patternedradiation beam onto a target portion of the substrate, a fluid supplysystem which is constructed to supply a fluid between the substrate anda downstream end of the projection system, and a position controller tocontrol a position of the fluid supply system, a position quantity ofthe substrate being provided to the position controller, wherein theposition controller is arranged to determine a desired position of theliquid supply system by adding a position offset to the positionquantity of the substrate, and position the liquid supply systemaccording to the desired position.

According to an aspect of the invention, there is provided an immersionprojection apparatus to project a beam onto a substrate, including asubstrate table constructed to hold the substrate, a projection systemconfigured to project a radiation beam onto a target portion of thesubstrate, a fluid supply system which is constructed to supply a fluidbetween the substrate and a downstream end of the projection system, anda position controller to control a position of the fluid supply system,a position quantity of the substrate being provided to the positioncontroller, wherein the position controller is arranged to determine adesired position of the liquid supply system by adding a position offsetto the position quantity of the substrate, and position the liquidsupply system according to the desired position.

According to an aspect of the invention, there is provided a devicemanufacturing method including projecting by a projection system apatterned radiation beam onto a target portion of a substrate, supplyingby a fluid supply system a fluid between the substrate and a downstreamend of the projection system, and controlling a position of the fluidsupply system, wherein the method further includes obtaining a positionquantity of the substrate, determining a desired position of the liquidsupply system by adding a position offset to the position quantity ofthe substrate, and positioning the liquid supply system according to thedesired position.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system used in a prior artlithographic projection apparatus;

FIG. 4 depicts a liquid supply system according to another prior artlithographic projection apparatus;

FIG. 5 depicts a liquid supply system according to still another priorart lithographic projection apparatus;

FIG. 6 depicts a substrate table and liquid supply system of alithographic apparatus according to the invention;

FIG. 7 depicts the substrate table and liquid supply system and a highlyschematic diagram of the control device to position the liquid supplysystem according to the invention;

FIG. 8A-F depict flow diagrams of embodiments of the lithographicapparatus, method and projection apparatus according to the invention;and

FIG. 9A-D depict examples according to the flow diagrams of FIGS. 8A-D.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g., UV radiation or DUV radiation).

a support structure (e.g., a mask table) MT constructed to support apatterning device (e.g., a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e., bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system.”

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD (not shown) for adjustingthe angular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g., an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g., so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g., after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

FIG. 6 depicts a detailed view of a part of the lithographic apparatusaccording to the invention. In particular, FIG. 6 schematically depictsa part of the projection system PS, a metrology frame MF (which servesas a mechanical reference), the liquid supply system, indicated here asLS, a substrate W held by substrate table WT, and a wafer stage shortstroke structure indicated as WS-SS. A surrounding structure of thesubstrate W is formed by a cover plate CP, which covers a gap betweenthe substrate W and other surrounding elements, such as a TIS(Transmission Image Sensor) sensor, a closing disc CD, etc. Thesubstrate W is positioned such that a target portion TP of a surfacethereof is illuminated via the projection system PS. The liquid supplysystem LS provides for an immersion liquid between the projection systemPS (more precisely a final projection lens of the projection system PS)and a part of the surface of the substrate W. It is to be noted thatFIG. 6 is not drawn to scale and provides a schematic presentation only.Therefore, distances between the various elements as well as proportionsthereof may not correspond to a physical reality. Further, it isremarked that although in the above description a liquid supply systemLS has been mentioned, in general such liquid supply system could supplyany type of fluid, thus including a liquid as well as a gas, thus theliquid supply system could, speaking in more general terms, also beidentified as a fluid supply system. Consequently, instead of animmersion liquid as referred to in this document, any suitable fluidthus comprising a liquid as well as a gas, could be applied.

As explained above, the substrate table WT is positioned so as toposition the target portion TP of the substrate W such that it coincideswith a focus plane of the projection system PS as good as possible. Inaccordance with an embodiment of the invention, the liquid supply systemis, as depicted in FIG. 6, tilted with respect to the projection systemto follow a tilting of the substrate table WT, and thus of the substrateW. As the liquid supply system LS is tilted in this advantageousembodiment to follow tilting of the substrate table WT, a gap betweenthe surface of the substrate W and the liquid supply system LS can bekept sufficiently constant so as to avoid a too large gap at one side ofthe liquid supply system which may result in leakage whilesimultaneously avoiding a too small gap at the other side of the liquidsupply system which may result in a crash between the liquid supplysystem and the substrate. According to the invention, other or furthercriteria for positioning the liquid supply system may be applied inaddition to or instead of the tilting. Examples thereof will bedescribed in the below.

The operation of the lithographic apparatus according to the inventionwill now be described with reference to FIG. 7.

FIG. 7 highly schematically shows the projection system PS, liquidsupply system LS, substrate W and substrate table WT, which may beidentical to the one's depicted and described with reference to FIG. 6.The position of the substrate table WT, or more precisely in someembodiments the position of the substrate table short stroke WS-SSrelative to, e.g., the metrology frame MF or the projection system PS iscontrolled by a position control loop comprising a position sensor PS tosense a position of the substrate table WT and a substrate tableposition controller CON_(W) to drive an actuator (not shown) to positionthe substrate. The controller CON_(W) and the position sensor PS in thisembodiment form a close loop control loop. A position setpoint SET_(W)is provided to the control loop thus providing a setpoint, i.e., adesired position for the substrate table WT. The position setpoint mayhave been determined in an advantageous embodiment by determining alevel (height) map of the substrate W, thus measuring a height of thesubstrate W at a plurality of positions thereof. The height map can nowbe used to determine a suitable setpoint for positioning the substrate Wto be able to match a position of the target portion of the substratewhich is to be irradiated as good as possible with the focus plane ofthe projection system PS. This positioning of the substrate W mayinvolve a translation in a vertical direction, e.g., az-direction(indicated in FIG. 7), and/or may involve a tilting of thesubstrate w.r.t. in a plane perpendicular thereto, thus w.r.t. a planewhich is advantageously substantially parallel to a focus plane of theprojection system and/or around an axis perpendicular to an optical axisof the projection system. FIG. 7 further shows a position controller tocontrol a position of the liquid supply system. The position controllercomprises in this embodiment a position sensor PS_(L) to determine aposition of the liquid supply system and a controller CON_(L) to drivean actuator (not shown) to position the liquid supply system. Theposition sensor PS_(L) and the controller CON_(L) which are comprised inthis advantageous embodiment of the position controller according to theinvention in this embodiment form a close loop control loop. Accordingto the invention a setpoint, thus a desired position of the liquidsupply system is determined by adding an offset to a position quantityof the substrate. The position offset is in FIG. 7 schematicallyindicated by PO. The position offset is added to the position quantityPQ which is provided by the setpoint generator to generate the setpointSET_(W) for the substrate table. The position quantity may comprise atilting of the substrate or substrate table, the tilting, e.g., beingwith respect to the focus plane of the projection system. Advantages ofsuch tilting have been described above. Further, the position quantityPQ of the substrate table may comprise a position of the substrate tablein a direction perpendicular to the focus plane of the projectionsystem, thus in a Z-direction. This may be useful for some measurementsand/or irradiations in which the substrate table is moved in operationin a direction which at least partly coincides with the said axis, e.g.,to bring the surface of the substrate out of focus for some measurementsand avoiding a too large or too small gap between the liquid supplysystem LS and the substrate W. The offset may comprise a displacement(e.g., in a direction substantially perpendicular to the focus plane ofthe projection system), however may also comprise any tilting. In casethat the offset comprises, e.g., a displacement, the position quantityof the substrate may comprise a position as well as a tilting, thus theliquid supply system may be positioned according to an aspect of theinvention such that is offset in, e.g., a vertical (e.g., a Z-direction)with respect to the substrate while following a tilting of thesubstrate, hence obtaining the advantageous effects as described withreference to FIG. 6.

In an embodiment, the position offset comprises a substantially fixedoffset value during a scan of the substrate, thus providing for anaccurate following by the liquid supply system LS of the surface of thesubstrate W, while at the same time preventing complex calculations forposition dependent offsets etc. The substantially fixed offset value maycomprise a substantially fixed offset value in the Z-direction, thusadvantageously in the direction perpendicular to the focus plane of theprojection system, thus ensuring a fixed, predetermined gap between theliquid supply system and the substrate. This advantageous embodiment isadvantageously combined with the position quantity comprising therotational position, thus providing for an offset in the Z-direction,thus a gap in Z-direction, while following a tilting of the substratetable by the liquid supply system; thus resulting in not only a fixeddifference averaged over a surface of a portion of the substrate whichfaces the liquid supply system, but also eliminates as much as possibledifferences in the gap at edges of the liquid supply system. In otherwords, the position quantity comprises the rotational positions aroundthe X and Y axes being perpendicular to the Z axis; while keeping the Zaxis itself fixed. Thus, the distance between the surface of a portionof the substrate which faces the liquid supply system and the liquidsupply system is kept constant in Z-direction, while the substrate tabletilting is followed by the liquid supply system. This way, local tiltvariations on the wafer do not result in gap variations at the edges ofthe liquid supply system. The relative Z-distance between liquid supplysystem and substrate is be kept constant and only the large (average)global Rx/Ry tilt of the substrate is followed by the liquid supplysystem. Small slit related, but high frequent substrate topology is nottracked by the liquid supply system, because this would result in tolarge gap variations of liquid supply system (ca. 100×100) that has muchlarger dimensions than the slit (ca. 20*32 mm).

In the advantageous embodiment wherein the projection offset comprisesthe substantially fixed offset value during the scan of the substrate,it is further possible that the substantially fixed offset value isdetermined for a substrate by adding at least a part of a thicknessparameter of the substrate to a nominal offset value. In this way, acompensation is provided when the position quantity also comprises theposition of the substrate table in the Z-direction, as in that case athicker substrate would result in a lower setpoint for the substratetable and thus in a smaller gap between the liquid supply system and thesubstrate table. To compensate for this effect, at least a part of thethickness parameter of the substrate may be added to the nominal offsetvalue, thus in case of a thicker substrate, e.g., increasing the offsetin the Z-direction, while in case of a thinner substrate decreasing theoffset. This advantageous embodiment does result in a more equal gapbetween the liquid supply system and the substrate for a thick substrateas well as for a thin substrate. The thickness parameter of thesubstrate comprises in a practical embodiment a mean value, e.g., anaverage value, of a maximum and a minimum thickness of the substrate.

The position controller may be arranged to keep the fluid supply systemat a fixed position during a scan of the substrate. In this advantageousembodiment, the position quantity of the substrate may comprise anaverage position of the substrate table in the direction perpendicularto the focus plane of the projection system, thus in the Z-direction. Anadvantage is that the liquid supply system during a scan does not movewith respect to the projection system. Therefore, no disturbing forcesby a movement of the liquid supply system will be transferred to theprojection system and to and reference to which the projection systemmay be connected, such as the metrology frame. Thus, any positioninaccuracies and projection inaccuracies by such disturbing forces maybe avoided. A further advantage is that no or only limited forces act onthe metrology frame MF as a result of displacements of the liquid supplysystem during the scan. In a practical embodiment the actuator oractuators to position the liquid supply system may act with reference tothe metrology frame MF thus causing reactive forces to the metrologyframe MF when displacing the liquid supply system or otherwise actuatingthe actuators.

The position controller is in a further advantageous embodimentconfigured to determine a momentary distance between a lower edge of theliquid supply system and a surface of the substrate. Further, theposition controller is in this embodiment configured to temporarilyadjust the offset when the distance reaches an extreme of apredetermined desired distance range. The desired distance range may beset so as to certify a minimum distance which provides a minimum safetydistance between the liquid supply system and the substrate whilecertifying a maximum distance which does not exceed a distance (and thusa gap between the liquid supply system and the substrate) which exceedsa value which could potentially lead to problems such as leakage, etc.The momentary distance may be determined with any kind of distancemeasuring device, in an embodiment, the momentary distance is determinedfrom a difference between a position of the substrate or the substratetable and a position of the liquid supply system. In this manner, asimple position controller can be implemented as the position controllerin this embodiment is only required to adjust the offset when thedistance reaches an extreme of the predetermined desired range.

In another advantageous embodiment, the position controller isconfigured to determine a distance between the liquid supply system andthe substrate and to issue a warning message when the distance exceeds apredetermined safety distance range. The warning messages may compriseany type of message including a digital code in a computer network aswell as any type of optical or acoustical warning or any otherindication. Thus, a safety message is generated when a predeterminedsafety distance range is exceeded.

A further group of embodiments will be explained with reference to FIG.8 and FIG. 9. In these embodiments, the positioned quantity comprises aheight function of a substrate height. Such a height function may eitherconsist of a table comprising a respective height for a respectiveposition on the substrate, or the height function may comprise amathematical function. The term height is to be understood as a level ofa surface of the substrate as positioned on the substrate table in adirection substantially parallel to the Z axis as referred to above. Theheight function may have been determined in practical embodiments foreach substrate anyway, as is required to determine an optimal positionof the substrate table for positioning a target portion of the substratesuch that it coincides with a focus plane of the projection systems asgood as possible. In the below, some advantageous embodiments comprisedin this group of embodiments will be discussed.

As depicted in FIG. 8A, in an embodiment the position controller isconfigured to determine in step 800 by way of the height function a bestfitting plane over a part of the substrate surface which is covered bythe liquid supply system. Then, in step 801 the desired position of theliquid supply system is determined by adding a predetermined height tothe height of the best fitting plane. The predetermined height maycomprise any offset advantageously it comprises a height offset whichcorresponds to an optimum height in case that the surface of thesubstrate would be completely flat which height may be in a range of30-150 micrometers. In this manner, a gap between the liquid supplysystem and the substrate is kept constant at such optimal value as wellas possible. This embodiment is further graphically depicted in FIG. 9Awhere LS represents the liquid supply system, W represents a surface ofthe substrate and WBF represents the best fitting plane for thesubstrate surface.

It is noted that in the embodiments as described wit reference to any ofFIGS. 8A-8F, as well as may be the case for any other embodimentdescribed in this document, the term height or height difference is tobe understood as a position or position difference along an axis whichis substantially perpendicular to the focus plane of the projectionsystem. Further, it is noted that not only the height of the bestfitting plane may be determined and used, but also a local tilt of thebest fitting plane. This may not only be applicable for the FIG. 8Aembodiments, but may be applicable for other embodiments also, inparticular but not limited to the FIG. 8B-8F embodiments.

In an embodiment, the positioned controller is configured to determinein step 810 by way of the height function a plane that keeps a minimumvalue of the height function positive over the part of the substratesurface which is covered by the liquid supply system. Then, in step 811a desired position of the liquid supply system is determined by addingthe predetermined height to the plane that keeps the minimum value ofthe height function positive. Thus a calculation similar to thecalculation of the above embodiment as described with reference to FIG.8A is performed, but instead of the best fitting plane, a plane iscalculated that keeps the minimum value of the height function positivehence resulting in a position that maintains at least a minimum gap,thus a minimum distance between the liquid supply system and the surfaceof the substrate. This embodiment is graphically depicted in FIG. 9B,where WPOS expresses the plane which is calculated for which the minimumvalue of the height function is positive.

In an embodiment, as depicted in FIG. 8C, the positioned controller isconfigured to in step 820 sum by way of the height function a heightover the part of the substrate surface which is covered by the liquidsupply system; divide in step 821 the summed height by a surface area ofthe part of the substrate surface which is covered by the liquid supplysystem to obtain a divided summed height; and determine in step 822 thedesired position of the liquid supply system by adding the predeterminedheight to the divided summed height. As a result, a constant volume isachieved. As an alternative to the summing of the height, it is alsopossible to perform an integration. With this embodiment, the positionand orientation of the liquid supply system is determined that keeps avolume of the liquid in the gap between the liquid supply system and thesubstrate substantially constant. In a further embodiment, as describedwith reference to FIG. 8D, the positioned controller is configured todetermine in step 830 by way of the height function an average heightover a part of the substrate surface which is covered by the liquidsupply system; and determine in step 831 the desired position of theliquid supply system by adding a predetermined height to the averageheight. As a result a minimum normal squeezing is achieved thus keepingthe volume contained in the gap constant while neglecting a tilt. Thisembodiment is further illustrated in FIG. 8D.

In an embodiment as depicted in FIG. 8E, the position controller isconfigured to determine in step 840 a disturbance force function overthe part of the substrate surface which is covered by the liquid supplysystem, the disturbance force function representing a force between thesubstrate and the liquid supply system in dependency on a distancebetween the substrate and the liquid supply system; integrate in step841, making use of the height function, the disturbance force functionover the part of the substrate surface which is covered by the liquidsupply system; and determine in step 842 the desired position of theliquid supply system as a position where the integrated disturbanceforce function over the part of the substrate surface which is coveredby the liquid supply system, may be constant over, e.g., an applicableare of horizontal displacements of the substrate with respect to theliquid supply system. With this embodiment a minimal substrate ofsubstrate table disturbance force is obtained. The inventor has devisedthat the force between the substrate resp. substrate table and theliquid supply system is strongly non-linearly dependent on the distancebetween these parts, thus on the gap. Therefore, in the calculation asapplied in this embodiment, these non-linear characteristics are takeninto account.

In a still further embodiment, as depicted in FIG. 8F, the positioncontroller is configured to determine in step 850 the disturbance forcefunction over the part of the substrate surface which is covered by theliquid supply system, the disturbance force function representing theforce between the substrate and the liquid supply system in dependencyon the distance between the substrate and the liquid supply system;determine in step 851 a position dependent height offset from thedisturbance force function; determine in step 852 by way of the heightfunction the best fitting plane over the part of the substrate surfacewhich is covered by the liquid supply system; and determine in step 853the desired position of the liquid supply system by adding the positiondependent height offset to the height of the best fitting plane. In thisembodiment, a minimum acceleration force is obtained. For the liquidsupply system to follow a non-flat substrate table, or a non-flat heightprofile, a force on the liquid supply system is required to accelerateit. In this embodiment, an actuator force is reduced as a single valuevector is used for the liquid supply system position or position map.The liquid supply systems is thus required to follow a displacement ofthe substrate and/or the substrate table.

In the embodiments as described with reference to FIGS. 8A-8F, thedesired position is calculated for a plurality of positions of thesubstrate with respect to the projection system, thus resulting in aplurality of positions which might be expressed in a form of a table, afunction, a level map, etc. The desired position might be calculated fordiscrete position steps of the substrate table, however might also beexpressed in terms of a continuous function. The desired position in theembodiments described with reference to FIGS. 8A-8F may comprise aposition and/or a tilt of the liquid supply system.

In this document, the term position controller may refer to any type ofcontroller comprising, e.g., a microcomputer, microprocessor, or anyother type of digital processing device provided with suitable softwareinstructions. Further, the position controller may comprise dedicatedhardware such as dedicated electronics to perform the said functions.

Further, in this document the term position quantity of the substratemay comprise any position related quantity comprising, e.g., a portion,a velocity, an acceleration of the substrate and/or the substrate table,a tilting, angular velocity or angular acceleration of the substrateand/or the substrate table. The position quantity may be with respect toany suitable reference, e.g., the projection system, the focus plane ofthe projection system, the metrology frame or any other fixed or movablereference of the projection system. The reference may be tangible (i.e.,comprising a physical structure) or comprise any mathematical or opticalreference, such as the focus plane.

Likewise, the desired position of the liquid supply system may comprisea position with respect to any reference and/or a tilting with respectto any reference.

The desired position may be determined in full or partly beforecommencement of an exposure of the substrate, however may also bedetermined in part or in full “on-the-fly,” i.e., during exposure and/ormovement of the substrate table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCD's), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion,” respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g., semiconductor memory, magnetic or optical disk) havingsuch a computer program stored therein.

The present invention can be applied to any immersion lithographyapparatus, in particular, but not exclusively, those types mentionedabove.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: a substrate table constructed tohold a substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate; a fluid supplysystem which is constructed to supply a fluid between the substrate andthe projection system; and a position controller to control a positionof the fluid supply system and to accept as an input, a positionquantity of the substrate, wherein the position controller is furtherconfigured to, determine a desired position of the fluid supply systemby adding a position offset to the position quantity of the substrate;and position the fluid supply system according to the desired position.2. The lithographic apparatus according to claim 1, wherein the positionquantity of the substrate comprises a quantity related to a tilt of thesubstrate table with respect to a focus plane of the projection system.3. The lithographic apparatus according to claim 1, wherein the positionquantity of the substrate comprises a position of the substrate table ina direction perpendicular to a focus plane of the projection system. 4.The lithographic apparatus according to claim 1, wherein the positionoffset comprises a substantially fixed offset value during a scan of thesubstrate.
 5. The lithographic apparatus according to claim 4, whereinthe substantially fixed offset value comprises a substantially fixedoffset value in a direction perpendicular to a focus plane of theprojection system.
 6. The lithographic apparatus according to claim 4,wherein the substantially fixed offset value is determined for asubstrate by adding at least a part of a thickness parameter of thesubstrate to a nominal offset value.
 7. The lithographic apparatusaccording to claim 6, wherein the thickness parameter of the substratecomprises a mean value of a maximum and a minimum thickness of thesubstrate.
 8. The lithographic apparatus according to claim 1, whereinthe position controller is configured to keep the fluid supply system ata fixed position during a scan of the substrate, the position quantityof the substrate comprising an average position of the substrate tablein a direction perpendicular to the focus plane of the projectionsystem.
 9. The lithographic apparatus according to claim 1, wherein theposition controller is further configured to determine a momentarydistance between the liquid supply system and the substrate, and totemporarily adjust the offset when the distance reaches an extreme of apredetermined desired distance range.
 10. The lithographic apparatusaccording to claim 1, wherein the position controller is furtherconfigured to determine a distance between the liquid supply system andthe substrate and to issue a warning message when the distance exceeds apredetermined safety distance range.
 11. The lithographic apparatusaccording to claim 1, wherein the position quantity of the substratecomprises a height function of a substrate height.
 12. The lithographicapparatus according to claim 11, wherein the position controller isconfigured to: determine by way of the height function a best fittingplane over a part of the substrate surface which is covered by theliquid supply system; and determine the desired position of the liquidsupply system by adding a predetermined height to the height of the bestfitting plane.
 13. The lithographic apparatus according to claim 11,wherein the position controller is configured to: determine by way ofthe height function a plane that keeps a minimum value of the heightfunction positive over the part of the substrate surface which iscovered by the fluid supply system; and determine a desired position ofthe fluid supply system by adding the predetermined height to the planethat keeps the minimum value of the height function positive.
 14. Thelithographic apparatus according to claim 11, wherein the positioncontroller is configured to: sum by way of the height function a heightover the part of the substrate surface which is covered by the fluidsupply system; divide the summed height by a surface area of the part ofthe substrate surface which is covered by the fluid supply system toobtain a divided summed height; and determine the desired position ofthe fluid supply system by adding the predetermined height to thedivided summed height.
 15. The lithographic apparatus according to claim11, wherein the position controller is configured to: determine by wayof the height function an average height over a part of the substratesurface which is covered by the fluid supply system; and determine thedesired position of the fluid supply system by adding a predeterminedheight to the average height.
 16. The lithographic apparatus accordingto claim 11, wherein the position controller is configured to: determinea disturbance force function over the part of the substrate surfacewhich is covered by the fluid supply system, the disturbance forcefunction representing a force between the substrate and the fluid supplysystem in dependency on a distance between the substrate and the fluidsupply system; integrate, making use of the height function, thedisturbance force function over the part of the substrate surface whichis covered by the fluid supply system; and determine the desiredposition of the fluid supply system as a position where the integrateddisturbance force function over the part of the substrate surface whichis covered by the fluid supply system, is constant.
 17. The lithographicapparatus according to claim 11, wherein the position controller isconfigured to: determine the disturbance force function over the part ofthe substrate surface which is covered by the fluid supply system, thedisturbance force function representing the force between the substrateand the fluid supply system in dependency on the distance between thesubstrate and the fluid supply system; determine a position dependentheight offset from the disturbance force function; determine by way ofthe height function the best fitting plane over the part of thesubstrate surface which is covered by the fluid supply system; anddetermine the desired position of the fluid supply system by adding theposition dependent height offset to the height of the best fittingplane.
 18. An immersion projection apparatus to project a beam onto asubstrate, comprising: a substrate table constructed to hold thesubstrate; a projection system configured to project a radiation beamonto a target portion of the substrate; a fluid supply system which isconstructed to supply a fluid between the substrate and the projectionsystem; and a position controller to control a position of the fluidsupply system, a position quantity of the substrate being provided tothe position controller, wherein the position controller is configuredto: determine a desired position of the fluid supply system by adding aposition offset to the position quantity of the substrate; and positionthe fluid supply system according to the desired position.
 19. A devicemanufacturing method comprising: projecting by a projection system apatterned radiation beam onto a target portion of a substrate; supplyingby a fluid supply system a fluid between the substrate and theprojection system; and controlling a position of the fluid supplysystem, the controlling comprising obtaining a position quantity of thesubstrate and determining a desired position of the fluid supply systemby adding a position offset to the position quantity of the substrate;and positioning the fluid supply system according to the desiredposition.